This invention generally relates to the fabrication of micromachined ultrasonic transducers. In particular, the invention relates to the fabrication of ultrasonic transducer arrays on CMOS wafers.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein. The systems resulting from such micromachining processes are typically referred to as “micromachined electromechanical systems” (MEMS).        
The cMUTs are usually hexagonal-shaped structures that have a membrane stretched across them. This membrane is held close to the substrate surface by an applied bias voltage. By applying an oscillatory signal to the already biased cMUT, the membrane can be made to vibrate, thus allowing it to radiate acoustical energy. Likewise, when acoustic waves are incident on the membrane the resulting vibrations can be detected as voltage changes on the cMUT. A “cMUT cell” is the term that will be used herein to describe a single one of these hexagonal “drum” structures. The cMUT cells can be very small structures. Typical cell dimension are 25–50 microns from flat edge to flat edge on the hexagon. The dimensions of the cells are in many ways dictated by the designed acoustical response. It may not be possible to create larger cells that still perform well in terms of frequency response and sensitivity desired.
Ultrasonic probes have been designed based on cMUT technology. In one known design, multiple cMUT cells are grouped together and the electrodes of cells in a particular group are hard-wired together to create larger transducer elements. One can form still larger elements, e.g., linear elements, by electrically connecting elements (i.e., so-called “subelements” comprising groups of hard-wired cMUT cells) together using a switching network. The larger elements can be reconfigured by changing the state of the switching network. However, the elements consisting of only one set of CMUT cells all hard-wired together cannot be reconfigured.
In accordance with one proposed architecture, each element comprises a multiplicity of hexagonal MUT cells arranged in a honeycomb pattern with the electrodes on the membranes hard-wired together. The outer ring of MUT cells in each element forms another hexagon. These elements can be reconfigured to form larger elements using a switching network. An array of such smaller elements can be integrated with conventional metal oxide semiconductor (CMOS) switches and preamplifier/buffer circuits onto a silicon wafer to provide reconfigurable beamforming elements. MEMS technology enables the realization of a two-dimensional cMUT array that resides on top of the CMOS electronics.
In accordance with a known method of manufacture, a pre-fabricated CMOS wafer is planarized prior to commencing the cMUT fabrication process. The CMOS wafer comprises an array of cells, each cell being composed of circuit elements that are used to provide required functions to its associated cMUT element locally. Connections between the plane of the CMOS cell matrix and the plane of the cMUT element array can be accomplished vertically.
Lithography is typically used in the fabrication of MEMS devices. The process typically involves the transfer of a pattern to a photosensitive material by exposing selected areas to a source of radiation such as light. The photosensitive material undergoes a change in its physical properties when exposed to radiation. Typically a mask is used that allows light to pass through and impinge only upon selected regions of the photosensitive material. In lithography for micromachining, the photosensitive material is typically a material (i.e., a photoresist) whose chemical resistance to developer solution changes when exposed to radiation of a specific wavelength. The developer solution is used to etch away one of the two regions (exposed or unexposed). A photosensitive layer can be used as a temporary mask when etching an underlying layer, so that the pattern can be transferred to the underlying layer. The photosensitive layer may also be used as a template for patterning deposited material.
In the fabrication of MEMS devices, different layers of the structure being fabricated must be aligned with each other. Each mask should have fiducial, i.e., alignment, marks, which are aligned with corresponding fiducial marks on the previously patterned layers, so that the corresponding layer can be registered with the other layers. The alignment mark on a mask may be transferred to the wafer, allowing an alignment mark on a subsequent mask to be aligned with the alignment mark on the wafer.
Mask making typically comprises layout and pattern transfer to the mask. The term “layout” refers to the process of defining the pattern that will appear on the mask, which in turn defines the geometry of the device being fabricated. Layout is typically performed in a graphical editing tool that manipulates a file containing layers of patterns. Each layer represents a respective mask. The layout tool allows the user to view and edit all of the layers together or selected layers. The pattern defined during layout must then be transferred to an optically opaque mask coating on an optically transparent mask substrate.
To fabricate a cMUT layer on top of a CMOS layer, an appropriate mask must be made using a conventional layout tool. In the case of a honeycomb pattern of hexagonal CMUT elements, there exist three natural axes of symmetry oriented at 60° relative to each other. The natural way to route signal and control lines in this coordinate system is along the axes of symmetry. In a rectilinear array of CMOS devices the natural axes of symmetry are mutually orthogonal. In this case, the natural way to route signal and control lines is along one of the orthogonal axes. While non-orthogonal lines can be drawn in standard CMOS processes, this can increase the incidence of defects and complicates mask production. When integrating cMUT devices that are distributed in a hexagonal or honeycomb grid on top CMOS devices that are distributed in a rectilinear grid, mismatch of unit elements occurs.
There is a need for methods of aligning a hexagonal grid of cMUT elements with a rectilinear grid of CMOS cells during micromachining. In particular, each hexagonal cMUT element must match up with its respective rectangular CMOS cell.